Digital optical systems that employ arrays of optical logical devices require an array of high intensity light beams in order to set and query the logic values of the devices. At the present time, laser diode arrays neither provide the beam quality nor have the appropriate configuration to generate a beam spot array that could adequately operate such an optical system. Binary phase gratings are diffractive optical elements that, in combination with imaging optics, produce a set of spots. These binary phase gratings (BPG), also referred to as Dammann gratings and described in H. Dammann and K. Gortler, "High-Efficiency In-Line Multiple Imaging by Means of Multiple Phase Holograms", Opt. Comm., 3, 312-315 (1971), and H. Dammann and E. Klotz, "Coherent Optical Generation and Inspection of Two-Dimensional Periodic Structures", Optica Acta 24, 505-515 (1977), are usable to produce an array of uniform intensity spots from a single collimated laser beam in the manner disclosed in U. Killat, G. Rabe, and W. Rave, "Binary Phase Gratings for Star Couplers with High Splitting Ratio", Fiber and Integrated Optics 4, 159-167 (1982).
In order for digital optical systems to move from the realm of theory to practical prototype, a number of issues related to spot array generation must be addressed. The issues associated with spot array generation fall primarily into the categories of theoretical design concerns and fabrication methods and tolerances and are summarized as follows: 1) design--matching the spot array with the associated system function and configuration; 2) uniformity--assuring that the relative intensities of the desired spots meet system tolerances; and 3) efficiency--optimizing usage of the illuminating laser beam power.
Design issues have primarily been centered on the ability to calculate solutions for large arrays of spots. Since the number of transitions between the two levels of a binary phase grating for a specific solution is proportional to the number of spots, the complexity of designing a large array escalates quickly. Because of this complexity the optimization programs and associated computational resources can quickly become a severe constraint. However, methods have been demonstrated to overcome this constraint as disclosed in F. B. McCormick, "Generation of Large Spot Arrays from a Single Laser Beam via Multiple Imaging with Binary Phase Gratings", Opt. Eng. 28, 299-304 (1989).
Intensity uniformity is the primary limitation of spot arrays now confronting optical system architects. Spot intensity uniformity is determined by requirements on the operation of the optical logic devices. Critical biasing and/or contrast differences between interconnected logic devices will often dictate these tolerances. Non-uniform intensities are primarily introduced by limitations of the fabrication process and may be impractical to control in many configurations.
Further, the phase grating must be able to maximally utilize the energy of the impinging laser beam by efficiently distributing the light to the desired spots. This is critical since the speed of the system is influenced by the energy available per optical device and economical high power laser sources are not currently available.
The typical binary phase grating of the prior art, referred to herein as the standard grating solution, forms an odd number of spots. An example of a spot array generated by a standard solution is shown in FIG. 1. Note that the spot array of FIG. 1 is a two-dimensional array of 25 spots comprising lines of five spots in one dimension and lines of five spots in a second dimension. More generally, a two-dimensional array of N.times.M spots comprises one-dimensional arrays (lines) of N spots in one dimension and one-dimensional arrays (lines) of M spots in a second dimension. The standard spot array design has a high intensity spot central order surrounded by uniform intensity sets of both positive and negative order spots. Symmetries inherent in the BPG design require that each positive and negative order pair have matching intensities. Therefore, only the central order spot can be influenced individually. Elimination of the central order from the standard solution in the manner disclosed in the Killat et al. paper, creates an array with an even number of spots; however the spatial regularity of the array is unfortunately destroyed. This is particularly significant in applications such as the optical crossover network disclosed in the U.S. patent application Ser. No. 07/349,281 of T. J. Cloonan et al. filed on May 8, 1989, allowed on May 20, 1991, now U.S. Pat. No. 5,077,483 issued Dec. 31, 1991 and assigned to the assignee of the present invention, where the use of each device of an array of optical devices is important in achieving the reduced blocking characteristics of the network. The elimination of the central order spot in the Killat arrangement means that the spacing between the two first order spots is twice as large as the spacing between the other spots. Therefore, use of the Killat arrangement in the Cloonan optical crossover network to illuminate arrays of equally spaced optical devices would render the devices corresponding to the suppressed central order unusable.